ANALYSES BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY OF BA CONCENTRATIONS IN WATER AND ROCK SAMPLES - COMPARISON BETWEEN ISOTOPE-DILUTION AND EXTERNAL CALIBRATION WITH OR WITHOUT INTERNAL STANDARD

Barium concentrations in various standards and natural samples are det ermined by inductively coupled plasma mass spectrometry (ICP-MS) using isotope dilution and using external calibration with or without inter nal standard. First, the measured Ba isotopic ratios confirm the exist ence of a mass bias, which is shown to follow a linear law and which i s the same in unknown natural samples (rocks, river waters, rain water s and sea waters) and in standards (river water, rock samples and synt hetic solutions) both in matrix-rich samples as well as in diluted sam ples. As it is not matrix dependent and it is stable on a daily time-s cale, approximate correction can be made. Second, Ba concentrations ob tained with isotope dilution and with external calibration with an int ernal standard show excellent agreement (within +/-2 %) for geostandar ds as well as for unknown natural samples. This result implies that in dium, as an internal standard for Ba analysis, is very effective for t he correction of matrix effects, whatever the matrix and signal variat ions. Concentrations calculated by isotope dilution and by simple exte rnal calibration are, however, in agreement to within +/-6%, which ind icates that matrix effects on Ba are small.